The present invention relates to the field of magnetic resonance ("MR") investigation of matter, and more particularly to a radio frequency (RF) shield for use therein.
The present invention finds particular application in conjunction with nuclear magnetic resonance (MR) imaging of the living body for medical diagnosis purposes and will be described with particular reference thereto. This description will emphasize the need to protect portions of the body of the patient from the radio frequency (RF) energy used in MR equipment and its effects and the need to protect the receiver coils used in the MR equipment from undesired signals. However, it is to be appreciated that the present invention has utility at present, and may in the future have further utility, in other applications using MR to investigate, locate or analyze animate or inanimate objects, such as MR spectroscopy, material analysis, explosive detection, well logging and the like.
During MR examination, it is often necessary to continuously monitor the patient, especially where the patient is sedated, aged, or infirmed. Where pulse oximetry is used for continuously monitoring the patient, three factors require consideration: safety of the patient, interference to the MR image, and interference to related equipment used proximally to the patient during the MR examination, such as a pulse oximeter. Patient safety problems relate to the possibility of RF burns at the points where the sensor attaches to the patient or where the electrical cables may touch the patient. The patient cable can act as an antenna to conduct RF energy from the MR apparatus to the patient, causing small, but possibly deep RF burns. Even where the pulse oximeter sensor is used in conjunction with a conventional uninsulated RF shield, a variety of arduous, time-consuming and burdensome precautions must be taken including the use of special cables, maintenance of the cables without loops or coils, periodic checking of the skin under the sensor for heating or reddening, and the like. MR images may also be degraded due to interference resulting from electronic signals induced in the patient cable by the oximeter or by external pick-up.
Similar considerations are obtained and similar precautions must be taken in connection with any of a variety of conductive articles or apparatus containing conductive articles, which may be employed proximally to the patient during MR examination--for example, for monitoring or obtaining measurements from the patient, for providing treatment to the patient, or the like. Treatments which may prove useful for patients undergoing MR examination and may also find need for an RF shield include the precise administration of contrast agents to enhance the diagnostic characteristics of lesions, drug delivery for pain management, chemotherapy and the like, administration of shock therapy simultaneous with or followed by patient evaluation using MR techniques, evaluation of organs for use as implants, treatment in association with the performance of sample biopsy and/or surgery. Measurements which may prove useful, in addition to the oxygen saturation and heart rate measurements obtained through pulse oximetry, include blood pressure, temperature, R-wave or EKG wave form, EEG or brain wave activity, respiration and carbon dioxide levels, analysis of drugs in the bloodstream, and numerous measurements of the patient and/or equipment associated with MR-guided sample biopsy and/or laser surgery. Accordingly, while the present invention will be described here primarily in connection with the use of a pulse oximeter sensor within the shield, the principles of the present invention are equally applicable when a different conductive article is employed as part of other monitoring or treatment apparatus or even where the conductive article is absent.
In MR imaging for medical diagnosis, many situations exist where it is desirable to provide local shielding for a patient undergoing an examination. In some cases it is desired to eliminate the excitation of nuclei and/or the detection of the signal in certain regions of the body. In this manner, specific artifacts and/or noise normally present in the magnetic resonance image may be reduced or eliminated completely. In other situations, it is desired to attach devices for monitoring or providing life support to the patient during the examination (e.g., a pulse oximeter). In this case, the shield must block RF signals emanating from the device, the leads of the device and/or device attachments in order to eliminate the detection of these signals by the receiver coils. It may also be required that the shield at the same time block RF signals generated by the MR imaging equipment which would otherwise interfere with the proper operation of the attached device.
In the past, shielding requirements described above have been attempted to be satisfied using aluminum foil as a conductive shield. In one instance signals from the patient's arms were eliminated from MR images using aluminum foil sheets wrapped around the arms. In another instance a monitoring device was successfully operated while patients were undergoing MR examinations using aluminum foil wrapped around a sensor attached to the patient's hand.
A conductor, such as aluminum foil, located in a RF environment provides electromagnetic shielding in part due to the existence of eddy currents which are established on the surface of the conductor. The magnitude and distribution of the currents induced on the conducting surface depend directly upon specific details of the existing RF environment as well as the geometry and material properties of the conducting surface. As the induced currents may lead to large voltages on the surfaces of the conductive shield relative to the patient or the surroundings, arc discharges may occur which can damage the patient or electronic equipment. RF induced currents may also lead to dangerous temperature elevations of the oximeter sensor, cables and/or conductive shield Indeed, there has been at least one reported incident of a severe finger burn caused by using a pulse oximeter for monitoring during MR imaging.
Thus, while aluminum foil is inexpensive and readily available, its use as a conductive shield in the MR environment can be unsafe for the patient, lead to damage of equipment, and produce misleading or unreliable results. Therefore, there remains a need in the field of MR examination for a safe, reliable and effective RF shield.
Accordingly, it is an object of the present invention to provide an RF shield for use on or around a sample undergoing MR examination which electrically blocks RF signals from the MR equipment in a safe, reliable, and effective manner.
Another object is to provide such a shield which electrically protects selected portions of the body of the patient from the RF energy used and its effects in a safe, reliable, and effective manner.
A further object is to provide an embodiment in which such a shield may incorporate at least a portion of an electrically conductive article, such as a sensor used for monitoring the functioning of a patient's body.
It is also an object of the present invention to provide such an embodiment in which the shield protects the conductive article from the RF energy and its effects.
It is another object to provide such an embodiment in which the shield blocks undesirable RF signals resulting from the presence of the conductive article in order to eliminate the detection of these signals by the MR examining apparatus.
It is yet another object to provide such a shield which thermally protects a selected portion of the body of the patient from the RF energy used and its effects in a safe, reliable, and effective manner.
It is a further object to provide such a shield which is of simple, rugged, and economical construction.